Interpretive Summary: Foliar diseases are a serious problem for alfalfa management in all areas where alfalfa is grown. Defoliation due to foliar diseases has been measured from 3-71% depending on time of year, environmental conditions, and locale. In addition to yield loss, foliar diseases can reduce seed yields and forage quality. Fungicide treatments are cost-effective in only some years and locations. Recently, genetically modified alfalfa that is tolerant to the herbicide glyphosate was approved for sale in the U.S. that expresses an enzyme that is not affected by glyphosate. However, fungal plant pathogens have an enzyme that is inhibited by glyphosate. In glyphosate-tolerant alfalfa plants, pretreatment of plants with glyphosate protected plants from infection by the alfalfa rust fungus for 14 days when applied to plants with 8-10 leaves and for 21 days when applied to plants with 2-3 leaves. Glyphosate application also cured infections of the rust fungus for up to 10 days after the fungus infected the plants. The protective and curative activity is sufficient for protecting forage that is harvested every 25-30 days during the growing season. Control of foliar diseases that cause necrotic leaf spots was less effective, suggesting that the intimate association that is established by the rust fungus is needed for disease control by the fungicide. Alfalfa rust is most serious in fields seeded in the fall. Control of rust using glyphosate is compatible with current recommendations for fall weed control in glyphosate-tolerant alfalfa, providing alfalfa farmers with a new tool for reducing damage and yield loss from rust disease.

Technical Abstract:
Glyphosate, the active ingredient in Roundup herbicide, inhibits 5-enol-pyruvyl shikimate 3-phophate synthase (EPSPS), an enzyme found in plants, fungi, and bacteria. Plants engineered for glyphosate tolerance with a glyphosate-insensitive EPSPS take up and translocate the herbicide throughout the plant. In greenhouse experiments we found that application of glyphosate at the recommended field application rate completely controlled alfalfa rust (Uromyces striatus) on 4-week-old plants inoculated with the fungus 3 days after glyphosate treatment. Also, excellent control of rust was obtained when glyphosate was applied up to 10 days after rust spores, indicating that the herbicide has protective and curative activity. Glyphosate application to stubble and regrowth provided protection for up to 21 days when plants were inoculated with rust spores 3 days after treatment or for up to 14 days when spores infected plants 11 days after treatment. Treatment afforded some control for anthracnose, caused by Colletotrichum trifolii, a hemi-biotrophic pathogen, but not for spring black stem and leaf spot, caused by Phoma medicaginis, a necrotrophic pathogen. These results indicate that glyphosate could be used to help manage foliar diseases in glyphosate-tolerant alfalfa.